Tularemia is a serious, often lethal infectious disease caused by the intracellular bacterium Francisella tularensis. A sense of the innate immune responses that protect mammalian hosts from Francisella has recently emerged from the study of subspecies that do not cause human disease. However, many of these concepts have not been tested in experimental infection models involving the type A and B strains that constitute the major Francisella pathogens in healthy humans. The long-term objective of this proposal is to determine whether or not inflammasome activation in infected macrophages plays a role in limiting the intracellular survival and replication of virulent Francisella subspecies. Published preliminary findings suggest this may not be the case in the mouse, although apoptotic cell death does occur once bacterial burdens reach high levels. Specific aims of the current project include defining the mechanism by which type A and B Francisella strains regulate pyroptosome activation and the production of type I interferons. Both are important signals for inflammatory casapse-mediated cell death in infected macrophages. Cell culture and a mouse respiratory infection model will be used to define specific steps in inflammasome activation that are regulated by these pathogens and determine the significance of macrophage survival to the course of infection. A second major aim is to use a large collection of clinical Francisella isolates to determine whether the induction of macrophage death is the basis for recently described differences in virulence properties of the Francisella subspecies and subclades that infect humans. Thus, this research challenges an existing paradigm of innate immunity to Francisella, namely that the early death of tissue macrophages provides an important host defense against the pathogen by eliminating a pathogen replication site. Understanding the differences in virulence mechanisms used by experimental model organisms and human Francisella clinical isolates will provide a better understanding of potential interventions that might prevent or treat infections with this dangerous pathogen.